/*
 * Copyright (C) 2007 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.inject.util;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.base.Objects;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Sets;
import com.google.inject.Inject;
import com.google.inject.Injector;
import com.google.inject.Provider;
import com.google.inject.spi.Dependency;
import com.google.inject.spi.InjectionPoint;
import com.google.inject.spi.ProviderWithDependencies;

import java.util.Set;

/**
 * Static utility methods for creating and working with instances of
 * {@link Provider}.
 *
 * @author Kevin Bourrillion (kevinb9n@gmail.com)
 * @since 2.0
 */
public final class Providers {

    private Providers() {
    }

    /**
     * Returns a provider which always provides {@code instance}.  This should not
     * be necessary to use in your application, but is helpful for several types
     * of unit tests.
     *
     * @param instance the instance that should always be provided.  This is also
     *     permitted to be null, to enable aggressive testing, although in real
     *     life a Guice-supplied Provider will never return null.
     */
    public static <T> Provider<T> of(final T instance) {
        return new ConstantProvider<T>(instance);
    }

    private static final class ConstantProvider<T> implements Provider<T> {
        private final T instance;

        private ConstantProvider(T instance) {
            this.instance = instance;
        }

        public T get() {
            return instance;
        }

        @Override
        public String toString() {
            return "of(" + instance + ")";
        }

        @Override
        public boolean equals(Object obj) {
            return (obj instanceof ConstantProvider) && Objects.equal(instance, ((ConstantProvider<?>) obj).instance);
        }

        @Override
        public int hashCode() {
            return Objects.hashCode(instance);
        }
    }

    /**
     * Returns a Guice-friendly {@code com.google.inject.Provider} for the given
     * JSR-330 {@code javax.inject.Provider}. The converse method is unnecessary,
     * since Guice providers directly implement the JSR-330 interface.
     * 
     * @since 3.0
     */
    public static <T> Provider<T> guicify(javax.inject.Provider<T> provider) {
        if (provider instanceof Provider) {
            return (Provider<T>) provider;
        }

        final javax.inject.Provider<T> delegate = checkNotNull(provider, "provider");

        // Ensure that we inject all injection points from the delegate provider.
        Set<InjectionPoint> injectionPoints = InjectionPoint.forInstanceMethodsAndFields(provider.getClass());
        if (injectionPoints.isEmpty()) {
            return new GuicifiedProvider<T>(delegate);
        } else {
            Set<Dependency<?>> mutableDeps = Sets.newHashSet();
            for (InjectionPoint ip : injectionPoints) {
                mutableDeps.addAll(ip.getDependencies());
            }
            final Set<Dependency<?>> dependencies = ImmutableSet.copyOf(mutableDeps);
            return new GuicifiedProviderWithDependencies<T>(dependencies, delegate);
        }
    }

    private static class GuicifiedProvider<T> implements Provider<T> {
        protected final javax.inject.Provider<T> delegate;

        private GuicifiedProvider(javax.inject.Provider<T> delegate) {
            this.delegate = delegate;
        }

        public T get() {
            return delegate.get();
        }

        @Override
        public String toString() {
            return "guicified(" + delegate + ")";
        }

        @Override
        public boolean equals(Object obj) {
            return (obj instanceof GuicifiedProvider) && Objects.equal(delegate, ((GuicifiedProvider<?>) obj).delegate);
        }

        @Override
        public int hashCode() {
            return Objects.hashCode(delegate);
        }
    }

    private static final class GuicifiedProviderWithDependencies<T> extends GuicifiedProvider<T> implements ProviderWithDependencies<T> {
        private final Set<Dependency<?>> dependencies;

        private GuicifiedProviderWithDependencies(Set<Dependency<?>> dependencies, javax.inject.Provider<T> delegate) {
            super(delegate);
            this.dependencies = dependencies;
        }

        @SuppressWarnings("unused")
        @Inject
        void initialize(Injector injector) {
            injector.injectMembers(delegate);
        }

        public Set<Dependency<?>> getDependencies() {
            return dependencies;
        }
    }
}
